Recently, improvement of the fuel economy of motor vehicles and reduction of the weight of vehicle chasses have been deemed important. To reduce the weight, the usage of high strength steel sheet has been increasing. However, the higher the strength, the more difficult the formability can become. In particular, the steel material can fall in elongation. Further, depending on the member to be produced, there may be a few parts where burring is performed to expand a machined hole to form a flange. A demand has also risen for a hole expandability as possibly an important characteristic.
Therefore, to address such demand, Japanese Patent Publication (A) No. 2001-192768, Japanese Patent Publication (A) No. 2001-200338, Japanese Patent Publication (A) No. 2001-3150, Japanese Patent Publication (A) No. 2001-207235, Japanese Patent Publication (A) No. 2001-207236, Japanese Patent Publication (A) No. 2002-38248, Japanese Patent Publication (A) No. 2002-309334, and Japanese Patent Publication (A) No. 2002-302734 have been provided which describe an improvement in the hole expandability in TRIP steel or composite structure steel sheet via a technique for using tempered martensite and conducting annealing heat treatment twice.
In this way, a high strength steel sheet for which hole expandability is preferred may be increasingly provided hot dip zinc coatings. On the other hand, there may also be a demand for high hole expandability high strength steel sheet without hot dip zinc coatings. In addition, relatively soft steel sheet previously used for exterior panels of motor vehicles and steel sheet with extremely large deep drawability used for oil pans, etc. may be regularly produced.
To produce such diverse types of steel sheet stably and efficiently, with a conventional single-objective type of continuous annealing facility continuously annealing steel sheet or a continuous annealing hot dip zinc coating facility able to continuously treat steel from annealing to hot dip zinc coating by a series of facilities, a plurality of such facilities can be combined and passed through. This may provide a problem of additional construction of facilities, lengthening of the production time, and increase in the production costs.